Because of their high strength coupled with light weight, composite parts are being increasingly used as structural components in a variety of articles including aircraft structure. Typical composite part materials include glass or graphite fibers that are embedded in resins such as epoxy, phenolic or bismaleimide resins. The fiber and resin are formed into a structure that is then cured under elevated temperature and pressure in a mold or curing tool.
Composite parts destined to be used in the aircraft industry must meet exacting requirements as to their structural integrity. To meet these exacting requirements precise control of lay up, debulking and curing including precise control of temperature and pressure during curing is required.
One widely utilized system for forming structural composite parts utilizes what are termed "prepregs." These are sheets of fiber that have uncured or partially cured resin embedded therein. The individual plies of prepreg, laid one on top of the other, are positioned inside of a cure tool and subjected to heat and pressure to cure the prepreg into a composite material. Various apparatus has been utilized or proposed for initially positioning and then maintaining the prepreg in contact with the tool or mold during the curing cycle. Such apparatus have included press platens, rubber bladders, vacuum bags and trapped rubber molding systems. Each of these has inherent limitations.
Utilizing press platens to pressurize the prepregs during cure is limited to essentially planar parts because of restrictions imposed by geometry. Rubber bladders also are limited by geometrical considerations. U.S. Pat. Nos. 4,773,952 and 4,933,040 both describe composite vacuum bottles that are both laid up and directly cured on a male mandrel utilizing a rubber bladder that fits over the male mandrel to consolidate the structure.
Vacuum bagging is much more versatile and can be utilized for complex parts. In a like manner trapped rubber moldings can also be utilized for some complex parts. In vacuum bagging, plies of prepreg inside a tool are covered with appropriate bleed or barrier cloths. A final vacuum bag is placed over the assembly. Vacuum is applied to the inside of the vacuum bag to evacuate the inside of the bag. The assembly is then loaded into an oven or autoclave and curing is accomplished by simultaneous heating and pressurizing the part in the oven or autoclave.
In trapped rubber molding a closed container is utilized. A portion of the interior of the container includes a mold or die surface that defines the part surface. The plies of prepreg are located on the mold or die surfaces following by appropriate bleed cloths and/or barrier cloths and the like. The remainder of the interior of the container is then filled with a pre-shaped solid silicon rubber member. When subjected to heating the silicon rubber member expands and in doing so it forces the prepreg against the die or mold surface.
Aside from any other consideration, trapped rubber moldings are difficult to control because of excess pressures that may be generated inside of the curing tool. These pressures can exceed desired and safe limits and trapped rubber molding cure tools have been known to catastrophically fail due to excess pressure build up. A variety of expedients have been tried in order to eliminate this type of problem. These include down sizing the silicon rubber expansion member or utilizing powdered or gelled silicon members.
In addition to mechanical problems, both trapped rubber molding and vacuum bagging consistently yield parts which have defects at those areas defined by inside radii on the mold or cure tool. These defects include porosity, bridging, resin richness and delamination. Porosity occurs when gasses or other voids are incorporated into the cured structure. Bridging results when the plies span across inside radius contours of cure tool instead of fitting flush against these contour area of the tool. Delamination results from separation of the plies from one another within the composite structure. Resin richness results from excess resin migration to the outsides of bends, curves and radius areas of the composite structure. Depending upon the extent and severity of the defect, the part may have to be reworked or in a worse case scenario, scrapped. Most delamination defects and a certain percentage of the porosity defects require scrapping of the part. Other porosity defects, resin richness defects and bridging defects have to be reworked. Irrespective of whether or not the part has to be reworked or scrapped, time, materials and processing steps have been wasted. This results in increased part cost.
While we do not wish to be bound by theory, it is presently believed that most porosity, bridging, resin richness and delamination defects located at corner radii of composite structures result from one key cause --this cause is lack of fiber slippage. During curing in female tools, the ends of the fibers within the prepreg plies become trapped and can not slip. Both vacuum bags and trapped rubber moldings tend to trap the fibers at flat areas on either side of corner radii. Since the fibers become trapped and can not slip, during consolidation and compaction they can not move to completely fill the radius areas.
Lack of consolidation leads to porosity. Trapping of fibers fixes the fibers but allows the more fluid resin to move under heat and pressure to the outside of the radius area. This leads to resin richness at these outside areas. Bridging results when the totality of the plies are trapped and can not move while delamination results when some of the plies are trapped and others are not such that the fibers (and resin) that move are separated from other fibers that are trapped.
Heretofore various expedients have been attempted in order to eliminate the above delineated defects. To date the method that has yielded the least amount of defective parts requires excessive hand working of the individual plies into each and every corner radius of a female tool. This and other similar expedients that are labor intensive are expensive to implement. Such labor intensive steps while useful for one or two pre-production parts do not lend themselves to large, medium or even small sized production runs. However, even when such labor intensive steps are implemented, defects still are encountered due to differences in operator skill level and operator attentiveness to an individual part.
Heretofore structural defects, especially corner radius defects of composites structures cured in female tools, have been an inherent problem that has not been solved. Accordingly there is a long felt need for tooling and the processes to minimize defects in composite structures, especially those composite structures that are cured in female tools.